wxr importer: defer comment counts

Description

Comment counts are calculated and recalculated every time a comment is inserted. That's inefficient when there are multiple comments per post.

The enclosed patch implements and uses a wp_defer_comment_counting() function. It's based on wp_defer_term_counting().

I also removed the comment_exists() check in the importer, since in practice it always returns false (the post_exists() check means we only attempt to import comments when the parent post is new, and hence has no comments).

Lloyd pointed out the comment_exists() change made a poor assumption. It didn't support a common use case where a second import is run after a first pass to catch up on comments that were added during the transition period. The second patch fixes this: it does the comment_exists() check when the post already exists, and skips it when it does not.